Four United crew members needed to get on the flight in order to work another one in Louisville or else that flight would be canceled, airline spokeswoman Maddie King said.

When no one volunteered, the airline was forced into an "involuntary de-boarding situation," airline spokesman Charlie Hobart said.

United used a system that weighs a number of factors to determine which passengers would leave the flight, such as connecting flights and how long the delay will leave the customer at an airport, airline spokesman Charlie Hobart said.

United employees explained the situation to the man several times, Hobart said. When he refused they followed Department of Transportation protocol and called local law enforcement to forcibly remove him from the plane.

Passenger Tyler Bridges said it was easy to understand why there were no volunteers. It was Sunday night and the next flight was not until the following afternoon.

Bridges said two officers tried to calmly talk the man out of his seat before a third approached him in an aggressive manner. The officer told him he had to get off the plane, and when he resisted, the officer grabbed him out of his seat .

The man hit his head on an armrest and yelled that he was a doctor and that he was being profiled for being Chinese, Bridges said. The man also expressed that he would not give up his seat as he needed to see patients the next day. The scene left everyone unsettled, including children who started crying.

As video of the incident spread, United came under intense criticism for its handling of the situation, with people pledging to boycott the airline.

The backlash prompted CEO Oscar Munoz to issue a statement. He called the incident "upsetting" and apologized "for having to re-accommodate" customers.

"Our team is moving with a sense of urgency to work with the authorities and conduct our own detailed review of what happened. We are also reaching out to this passenger to talk directly to him and further address and resolve this situation," the statement said.

The Chicago Department of Aviation said in a statement that the incident "was not in accordance with our standard operating procedure and the actions of the aviation security officer are obviously not condoned by the Department."

That officer has been placed on leave effective today pending a review of the situation, the statement added.

What about the crew members?

The four crew members did indeed board the plane, and it wasn't pretty, Bridges said. Passengers berated them, told them they should be ashamed of themselves and embarrassed to work for this company.

"They just sat down quietly, it was super tense on the plane. Everyone was really unhappy after seeing this man pulled off," he said.

"I think United messed this up on the front end," Bridges added. "It shouldn't have gotten to the point where there's a man on the plane or four people on the plane that have to be removed after they've already taken their seat. If they were overbooked they should have only let people on the plane that were going to be able to leave on the plane."